Every Super Bowl championship team opens the following season with a mission to win another title. This one was not enough. There is a great thirst for another one. Don’t worry about the players getting complacent because they have more to prove.

So why, then, have teams won back-to-back Super Bowls only eight times in the 46-year history of the game? The 2004 Patriots were the last do so. In fact, 13 Super Bowl champions have failed to make the playoffs the following season – led by the Pittsburgh Steelers, who have been left out three times. The Giants, themselves, have responded with a non-playoff season twice (after XXI and XXV).

There are many rationales – some unique – for what happened to each of these defending champions. Rather than reviewing the past, let’s try to get a better understanding of the present.

Here are three reasons why you should expect the Giants to defend the Lombardi Trophy:

1. Big Blue D. The Giants have improved a defense that played championship-caliber football down the stretch last season. For starters, they’ll be bolstered by the return of front-line cornerback Terrell Thomas (knee surgery), who is strong upgrade from free agent departure Aaron Ross (Jaguars). The talented and versatile Keith Rivers was acquired from the Cincinnati Bengals to add to a youthful group of linebackers – Mark Herzlich and Jacquian Williams have much potential.

Impact pass rusher Osi Umenyiora is happy with his raise, he’s in a contract year, and says he’s healthier than he’s been in a few seasons. They’ve also added numbers at defensive tackle, thanks to veteran free agent Shaun Rogers and the return of second-year pro Marvin Austin from a pectoral injury.

2. Eli’s elite. Is there anybody who still doesn’t appreciate and/or respect Eli Manning as one of the best quarterbacks in the NFL? It doesn’t matter, because he’s of the mentality that there’s always improvement to be made – and his brother, Peyton, may be the only passer in the league who works harder and more closely with his own receiving corps.

Big plays and fourth-quarter comebacks are not only the product of talent, they require instincts and carefully crafted chemistry – things Manning never takes for granted. He will do wonders for the development of pro-ready second-round WR Rueben Randle (LSU).

3. Tom Coughlin and Co. Who knows what mantra the coach will use this year as he attempts to win his third Super Bowl since the 2007 season? But it’s guaranteed to be carefully thought out and worth mulling over because his brand of old-school mentality – combined with a respected staff – virtually assures the Giants that they will be properly prepared to win every week.

Here are three reasons why you should not expect the Giants to defend the Lombardi Trophy:

1. O-line? Oh, no! The pass protection was adequate, but another shuffling has taken place. David Diehl will flop to right tackle, where veteran Kareem McKenzie was allowed to explore free agency. This requires athletic fourth-year pro William Beatty (eye) to step up and fulfill the role he was drafted for as the starting left tackle – but, to this point, his finesse game has not inspired great confidence.

Veteran backup Kevin Boothe has the inside track as the starting left guard – he’s not very agile, but has proven effective, especially in the power running game. Veteran center David Baas (neck, knee) battled through injuries and did not play up to his potential during his first season with the team, but he must prove they were right to sign him in 2011.

2. Running on empty. Veteran Ahmad Bradshaw is the team’s only proven running back – thanks to Brandon Jacobs’ departure for the San Francisco 49ers – and he’s battled foot injuries throughout his career. Oft-injured D.J. Ware never been thought of as higher than third on the depth chart. Second-year pro Da’Rel Scott showed great quickness but questionable hands in limited touches as a rookie and first-round pick David Wilson (West Virginia) must improve his ball security skills and prove he can be counted on to pick up a blitz.

3. Off target. Can Victor Cruz be relied upon again after last year’s breakout season resulted in 82 catches for a team-record 1,536 yards and 9 TDs? How well will Hakeem Nicks bounce back from off-season surgery on his broken foot? Who will be the third receiver, now that the elusive and clutch Mario Manningham signed with the 49ers?

Domenik Hixon’s coming off a second ACL surgery, Jerrell Jernigan was overmatched as a rookie and fourth-year pro Ramses Barden is getting one final chance to prove he belongs. Injured TE Jake Ballard (knee), a clutch receiving threat, was claimed off waivers by the Patriots – does former Cowboy Martellus Bennett fill the void?